New species of Metynnis Cope, 1878 (Characiformes: Characidae) from the rio Paraguay basin, Mato Grosso State, Brazil

A new species of Metynnis is described from the rio Cuiabá and rio Manso drainages, in the upper rio Paraguay basin, Mato Grosso State, Brazil. This laterally compressed species, which rarely exceeds 140 mm in SL, is readily distinguished from other members of the genus Metynnis by having 100-110 lateral-line scales, 48-56 rows of scales above the lateral line, 32-36 circumpeduncular scales, 22-24 gill rakers, 38 vertebrae, singular color pattern, and relatively smaller snout, width, and head length.


Introduction
The genus Metynnis Cope, 1878 is widespread in South America, including the Amazon, Orinoco and Paraguay basins, rivers from northern and eastern Guyanas, and lower rio Paraná (Jégu, 2003).However, it recently invaded the upper rio Paraná basin after the construction of the Itaipu dam (Langeani et al., 2007).Currently, 14 valid species are recognized within the genus (Zarske & Géry, 1999;2008).The most recent phylogenetic study on the genus Metynnis suggests that it is included in the "piranha" clade with the genera Serrasalmus, Pristobrycon, Pygocentrus, Pygopristis and Catoprion (Ortí et al., 2008).
The upper rio Paraguay basin contains one of the largest floodplains in the world, the Pantanal, which consists of a wide sedimentary plain which extends from Central Brazil to Eastern Paraguay and Bolivia.The rio Cuiabá flows through two Brazilian States (Mato Grosso and Mato Grosso do Sul) and flows through the Pantanal into the rio Paraguay near the border between Brazil and Bolivia.One of the most important tributaries of the rio Cuiabá is the rio Manso, where a hydroelectric dam was built in late 1999.In the rio Paraguay basin Jégu & Ingenito (2007) registered four species of Metynnis: M. hypsauchen (Müller & Troschel, 1844), M. maculatus (Kner, 1858), M. mola Eigenmann &Kennedy, 1903 andM. otuquensis Ahl, 1923.During sampling carried out by the Núcleo de Pesquisas em Limnolgia, Ictiologia e Aqüicultura (Nupélia), of the Universidade Estadual de Maringá, in the region of influence of the Manso Reservoir, specimens of a new species of Metynnis were caught.This species is described herein.

Material and Methods
Measurements were taken point-to-point with a caliper to the nearest 0.1 mm, and counts under a stereomicroscope, both on the left side of the specimens whenever possible.Morphometrics were taken following Géry (1972), with the addition of three measurements: caudal-peduncle depth, length of supraoccipital process, and distance between end of supraoccipital process and dorsal-fin origin.Morphometric data of the body are presented in proportions of the standard length (SL) and data of head structures are presented in proportions of the head length (HL).Meristic data followed Zarske & Géry (1999).Vertebrae were counted in four x-rayed specimens (MZUSP 100626-holotype, NUP 2055 and MCZ 168058).In vertebral counts the fused PU1+U1 is considered a single bone, and the vertebrae incorporated into the Weberian apparatus are counted as four elements.Data presented in the diagnosis were obtained by comparative analysis of specimens (when available), original descriptions of all congeners and from the genus revision by Zarske & Géry (1999, 2008).Therefore, ranges provided in the diagnosis have minimum and maximum values recorded by Zarske & Géry (1999, 2008)  Dorsal-fin rays i,15-17 (mean = 16.5);not reaching adiposefin origin when adpressed; one forwarded spine anterior to dorsal-fin origin.Dorsal-fin origin at mid-body or a little behind of vertical through pelvic-fin origin; dorsal-fin margin truncate.Adipose fin well-developed; its base longer than distance between dorsal-fin insertion and adipose-fin origin; adiposefin origin before vertical through middle anal fin.Pectoral-fin rays i,10-13 (mean = 12.6); its distal margin convex, with anterior rays longer, forming roundish edge, not reaching vertical through dorsal-fin origin when adpressed.Pelvic-fin rays i,6-7 (mean = 6.7); its distal margin convex, with anterior rays slightly elongate, forming roundish edge, not reaching vertical through anal-fin origin when adpressed.Anal-fin rays iii,32-36 (mean = 34.1);with sexually dimorphic distal margin (see Fig. 3); unbranched rays well-developed and joined in structure similar to spine.Caudal-fin rays vii,17 (9+8); its distal margin bifurcate, with both lobes of similar length.

Color in alcohol.
Overall body background color brown, darker on head and dorsal portion of body.Body sides with numerous, roundish, dark, pupil sized spots, with illdefined contour, more concentrated at middle of body depth.Lateral spots sometimes coalesced and forming transversal bands (Fig. 2).Dark humeral blotch, with illdefined contour, behind opecular edge, above or on lateral line.Dorsal, adipose, pectoral, anal and caudal fins darkly pigmented, pigments more concentrated along its distal edge, forming narrow dark stripe.Anal fin of juveniles (less than 70.0 mm SL) with only a narrow dark margin.Pelvic fin without pigmentation.
Sexual dimorphism.Adult males of Metynnis cuiaba exhibit a lobule at the distal edge of the anal fin, and dorsal-fin rays are not elongated.Adult females have distal anal-fin margin somewhat straight and first dorsal-fin rays elongated, forming filaments apparently only during breeding period (Fig. 3).sexual dimorphism.Ontogenetic development was not evaluated either, resulting in the description of three different taxa based on juveniles, and both adult males and females of the same species (Zarske & Géry, 1999).In addition, a still more serious problem is the small number of specimens analyzed by Ahl (1923), when revising the genus (36 specimens for 18 species).Géry (1979) emphasized the requirement of a revision of that group, contesting the synonymy between M. fasciatus Ahl, 1931 and M. hypsauchen.Two decades later, Zarske & Géry (1999) published a new revision of the genus including all unconsidered characters by previous authors.As result, they recognized 11 valid species, among them M. fasciatus and M. hypsauchen, and synonymyzed other 13.From these, nine were described in Ahl's (1923) publication, besides other four: M. unimaculatus Steindachner, 1908, M. orbicularis (Steindachner, 1908), M. dungerni Ahl, 1923and M. ehrhardti Ahl, 1927. As Zarske & Géry (1999) analyzed only type specimens of described taxa and neglected to look at a more comprehensive sample with a wider geographic scope, they did not examine material of the species, described herein as M. cuiaba.This resulted in the recognition of only four species in the rio Paraguay basin Etymology.The species name, cuiaba, is given in reference to the type-locality of the new species, rio Cuiabá, tributary to rio Paraguai.A noun in apposition.

Distribution. Metynnis cuiaba is known from the upper
Remarks.Juvenile specimens (less than 70.0 mm SL) have a higher proportion ratio between body depth and SL, which diminishes during ontogenetic development (Fig. 5).

Discussion
The first comprehensive study on Metynnis was published by Ahl (1923), who recognized 18 nominal species.In the following decades, several of those species were synonymized in the revision of the genus by Gosline (1951), resulting in the recognition of only six valid species.However, according to Zarske & Géry (1999), both supramentioned authors did not investigated type-specimens of all described taxa, neither considered the breeding period of those species, despite those species display an accentuated (M.hypsauchen, M. maculatus, M. mola and M. otuquensis), all already described.
Metynnis cuiaba is from the Pantanal, a very wide floodplain, located in the rio Paraguai basin, which has the rio Cuiabá as one of its main headwater rivers.Despite of its well known biodiversity richness and high degree of endemism, only M. mola and M. maculatus had been previously recorded in that basin.
Ontogenetic development was analyzed as well in the new species, and a pronounced allometry was observed.A higher body depth to SL ratio in juveniles is supposedly more related with avoiding predation when juvenile.
According to Veríssimo et al. (2005), the rio Manso bears many species in common with the typical fish fauna from the   Pantanal and rio Paraguai, and amongst other 248 species caught and recorded during that project are Metynnis cuiaba and its congener M. mola.The other congener, M. maculatus, was identified only from previous collection specimens collected near the sampled region approximately 30 years ago (MZUSP 20429).The region influenced by the Manso Reservoir was sampled upstream to the Pantanal, including the rio Cuiabá and a great diversity of environments, such as headwaters, streams, rivers, lakes and the reservoir itself, situated along the rio Manso.It comprises a significant area, however relatively small when compared to the whole basin, thus M. maculatus might occur in environments which were not sampled by the survey carried out by Veríssimo et al. (2005).
Species of Metynnis have been studied for decades, however there are still misidentifications, mainly because of the high ontogenetic variability, the sexual dimorphism and the absence of identification keys, besides the existence of several nominal taxa of doubtful status.Perhaps further careful inventories in the rio Paraguai basin could reveal even additional new species of Metynnis.In spite of serrasalmin systematics are undergoing major revision and much remains to be done.Even the status of the family is still not a consensus.Some recent researchers still maintain the subfamily Serrasalminae (Jégu, 2003), while others recognize them as a distinct family, the Serrasalmidae (e.g.Calcagnotto et al., 2005).According to Freeman et al. (2007) future studies should include combined genetic data and careful documentation of specimens examined.

Fig. 4 .
Fig. 4. Partial map of Brazil and adjoining countries showing collecting sites of Metynnis cuiaba.Asterisk = type locality.Dot = distribution (symbols represent more than one collecting locality).

Fig. 5 .
Fig. 5. Ratio between standard length and body depth in juveniles and adults of Metynnis cuiaba.

Metynnis cuiaba, new species Figs. 1-5
Morphometric data presented in Table 1.Body deeply compressed, with rounded dorsal and ventral profiles.Highest body depth on vertical line passing through dorsal and pelvic fins origin.Predorsal distance longer than postdorsal.Dorsal profile of body convex from snout to dorsal-fin origin; dorsal fin with straight base, very inclined posteriorly; preadipose profile obliquely straight, slightly convex from adipose-fin origin to caudal-fin origin.Ventral profile convex, with series of 32-36 simple spines in ventral keel (mean = 34.8;23-26 + 9-10), followed by one or two bifurcate spines, never surpassing anus.Caudal peduncle short, much deeper than longer, with dorsal and ventral margins straight or slightly concave.Dorsal profile of head concave.Ventral profile of the head oblique, almost convex.Snout roundish in lateral view.Eye lateral.Mouth terminal with molariform teeth.Inner premaxillary row with five teeth, outer with two.Dentary with four anterior teeth.Branchial membranes joint together and free of isthmus.Twenty two to 24 (mean = 22.7; 10 + 12-14) tubercular, short and thick gill rakers; cerato-and epibranchials similarly long.